IEEE 802.11 is a set of Medium-Access Control (MAC) and Physical layer (PHY) specifications for implementing WLAN, also known as Wi-Fi, computer communication in the 2.4, 3.6, 5 and 60 GHz frequency bands. The specifications were created and maintained by the IEEE Standards Committee IEEE 802. The base version of the standard was released in 1997 and has had subsequent amendments. The standard and amendments provide the specification basis for wireless network products using the Wi-Fi bands.
Typically, LTE-U refers to a version of LTE operating in the unlicensed band. Such LTE deployment would use the unlicensed spectrum that is today generally used by Wi-Fi. There are two main approaches to LTE usages in unlicensed spectrum, LTE-U as stand-alone and License-Assisted Access via LTE (LAA-LTE). In the first approach, LTE transmits all its carriers in unlicensed spectrum, while in the second LAA LTE is used as a “secondary carrier” in the unlicensed spectrum but with a primary carrier in a licensed band.
LAA-LTE is one of the main work items for the 3GPP LTE Release 13 standard being proposed as a technology for operation on both licensed and unlicensed spectrum.
In an LAA-LTE deployment, the User Equipment (UE), which may also be called a wireless device, connects to an LTE network on a regular, licensed spectrum band, the so-called P-cell (Primary cell). Additionally, the UE may also be connected to the same network on an unlicensed spectrum, the S-cell (Secondary cell). There may be more than one S-cell, i.e., more than one carrier in the unlicensed band. The unlicensed carrier can be used to off-load the licensed carrier for data whenever it is available. In this way the licensed carrier can be used for robust control signaling and the unlicensed carrier mainly used to boost user-data rates in a best-effort fashion.
Listen-Before-Talk (LBT) is a protocol where the desired channel on the wireless medium is first sensed for any potentially interfering transmissions before a transmission begins. If the medium is found to be free, then the transmitter can start using it. Together with a back-off mechanism, an LBT-protocol potentially avoids collisions. The LBT protocols usually consist of a number of steps that include:                Listen to medium measure the received signal with procedures such as Clear-Channel Assessment (CCA).        Decision of medium busy/free based on energy detection and/or decoding of signal.        Start transmission if medium is free, or after a defined back-off period, the system starts the transmission.        
In order to sense if the channel is busy, one can average the energy over a period of time or look at instantaneous peaks. An average over time, which is in effect low-pass filtering, has less false positives than an instantaneous measurement and decision. On the other hand, the method takes longer since it has to sense the channel and average the energy over some period of time.
In a radio-access network with more than two nodes, there is the potential of so-called hidden nodes. FIG. 1 illustrates an example hidden node situation in a wireless network. This situation consists of some nodes (set A) that can hear, and can be heard by, some other set of nodes (set B). There is also a third set of nodes (set C), which can hear, and is heard by, set A. However, nodes in set B and set C cannot hear each other. This is a problem for nodes in set B and set C when assessing the channel in preparation for transmission to nodes in set A. They cannot hear on-going transmissions towards an A-node that is transmitted from nodes in the other set.
Existing solutions in LTE-U for doing channel assessment using LBT utilize a fixed power threshold in all situations to determine whether or not the channel is busy. This can be particularly problematic in a coexistence scenario with Wi-Fi and can lead to unwanted degradation of the Wi-Fi performance. This is a variant of the hidden node problem. The Wi-Fi transmitter performs channel assessment and deems the channel to be free. It then transmits and during this time an LTE-U node doing LBT will detect the channel as busy. However, the Wi-Fi receiver will shortly thereafter transmit an ACK/NACK without doing a new LBT. This is part of the Wi-Fi protocol and all Wi-Fi nodes that decode the preamble of the initial transmission will know to wait for the ACK/NACK to go through.
The LBT-mechanism employed by LTE-U units is constructed with defer periods such that the short gap between the initial Wi-Fi transmission and the ACK/NACK is not regarded as a free channel. However, a problem occurs when the power level of the ACK/NACK is lower than the power level of the initial Wi-Fi transmission. This would typically happen when the initial transmission is from the Wi-Fi access point (AP) since the Wi-Fi stations commonly transmit with lower power. The ACK/NACK may not be detected by the LTE-U, and hence, potentially overrun by an LTE-U transmission. The reverse situation is also possible, with the LTE-U unit close to the Wi-Fi station, thus sensing the initial transmission from the station but not being able to hear the ACK/NACK from the AP.
The severity of the problem lies in the fact that the initial Wi-Fi transmission is never acknowledged (although it was in all likelihood received correctly) because the ACK/NACK was interfered. This then causes the transmitting Wi-Fi unit to back off further by increasing the contention window, and thus further reducing the chance of the WiFi-transmission getting sent successfully.
A similar situation may occur in a wastage of spectrum scenario. FIG. 2 illustrates an example wastage of spectrum scenario in a wireless network 10. Specifically, FIG. 2 illustrates an example embodiment wherein a roof-mounted Wi-Fi Access Point (AP) 12 and a Secondary Cell (S-Cell) eNodeB 14 exhibiting low pathloss. eNodeB 14 and Wi-Fi AP 12 can bear each other 16. However a collision 18 may occur at eNodeB 14.
As depicted, a Wi-Fi station 20 may transmit on uplink 22. As stated, above Wi-Fi station 20 transits at a power that is less than −62 dBM. Accordingly, it may not be detected by LTE-U node 24, which cannot hear an LTE-UE transmission 22 that is less than −62 dBm. In such a scenario the whole, or part of, the UL transmission 22 can potentially collide with a transmission 26 from the LTE-U node 24, thereby rendering both the Wi-Fi transmission 22 and LTE-U transmission 24 undecodable which leads to wastage of spectrum.
In general terms, FIG. 2 illustrates the scenario where the LTE-U eNodeB 14 and the Wi-Fi AP 12 are co-located, or at least close enough that they can hear each other. The links between the eNodeB 14 and the LTE-U node 24, and the Wi-Fi AP 12 and the Wi-Fi station 20, respectively, may be good. However, if the eNodeB 14 employs a CCA-ED threshold of −62 dBm, eNodeB 14 may not detect the Wi-Fi UL transmission 22 and, consequently, interfere the Wi-Fi reception at Wi-Fi AP 12 when eNodeB 14 transmits a downlink (DL) transmission 28 to the LTE-U node 24. This may also result in interference of the DL reception at the LTE-U node 24 if the LTE-U node 24 and the Wi-Fi station 20 can hear each other. Note also that a Wi-Fi DL transmission 30 may also be impacted if the corresponding ACK/NACK for the Wi-Fi DL transmission 30 gets caught in this scenario.
Problems may also exist in the reverse scenario. Specifically, the Wi-Fi AP 12 may be unable to detect the LTE-U UL transmission 26 when the power level of the transmission is below −62 dBm. Hence, the Wi-Fi AP 12 may transmit donwlink transmission 30 when the LTE-U node 24 transmits uplink transmission 28, which may result in interference at the eNodeB 14. Interference may also result at the Wi-Fi station 12 if the Wi-Fi station 12 is located near LTE-U node 24.